This invention relates to the art of thermal heads for thermal recording which are used in various types of printers, plotters, facsimile, recorders and the like as recording means.
Thermal materials comprising a thermal recording layer on a substrate of a film or the like are commonly used to record images produced in diagnosis by ultrasonic scanning (sonography).
This recording method, also referred to as thermal recording, eliminates the need for wet processing and offers several advantages including convenience in handling. Hence in recent years, the use of the thermal recording system is not limited to small-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required is under review.
As is well known, thermal recording involves the use of a thermal head having a heater (glaze), in which heating elements comprising heat-generating resistors and electrodes, used for heating the thermal recording layer of a thermal material to record an image are arranged in one direction (main scanning direction) and, with the glaze urged at small pressure against the thermal material (thermal recording layer), the two members are moved relative to each other in the auxiliary scanning direction perpendicular to the main scanning direction, and the heating elements of the respective pixels on the glaze are heated by energy application in accordance with image data to be recorded which were supplied from an image data supply source such as MRI or CT in order to heat the thermal recording layer of the thermal material and form color, thereby accomplishing image reproduction.
In thermal recording, density unevenness on the recorded image brings about a reduction in the quality of the finished image, which may cause an important problem in applications requiring high quality image recording. Especially the aforementioned medical application requires high quality images. Density unevenness also prevents image observation which will cause an important problem leading to an erroneous diagnosis. It is thus required that the thermal head is capable of recording high quality thermal images without density unevenness and having a reduced deterioration with the passage of time over an extended period of time.
Primary causes of the deterioration of the thermal head with the passage of time include variation in the properties of heating elements due to heat generation, and wear and corrosion (or wear due to corrosion) of the glaze.
A heating element of the thermal head usually comprises a heat-generating resistor and a pair of electrodes which energize the heat-generating resistor. The resistance value of the heating element varies with the time for heat generation and the energy for heat generation. Hence, the more the resistance value decreases, the more the amount of heat generation increases. The temperature of the heating element associated with the supplied energy for heat generation increases by the reduced resistance value, which brings about an increase in the image density.
The heating history which shows the total amount of heat generation, or the ratio of heat generation for the image recording performed is inherently different in each of the heating elements of the thermal head mounted in the thermal recording apparatus. Then, the amount of resistance variation is also different in each of the heating elements. Therefore, in the course of image recording, differences in the amount of resistance variation are produced among the respective heating elements, which gives rise to density unevenness on the recorded image in association with the differences.
The thermal head and the thermal material are moved relative to each other to perform recording, with the glaze thereon urged at small pressure against the thermal material. A protective film is formed on the surface of the glaze of the thermal head in order to protect the heating elements and other components. It is this protective film that contacts the thermal material during thermal recording and the heat-generating resistors heat the thermal material through this protective film so as to perform thermal recording.
The protective film is usually made of wear-resistant ceramics; however, during thermal recording, the surface of the protective film is heated and kept in sliding contact with the thermal material, so it will gradually wear and deteriorate upon repeated recording.
If the wear of the protective film progresses, density unevenness will occur on the thermal image or a desired protective strength can not be maintained and, hence, the ability of the film to protect the heaters is impaired to such an extent that the intended image recording is no longer possible (the head has lost its function).
Particularly in the applications such as the aforementioned medical use which require multiple gradation images of high quality, the trend is toward adopting thermal films with highly rigid substrates such as polyester films and also increasing the setting values of recording temperature and of the pressure at which the thermal head is urged against the thermal material. Under these circumstances, as compared with the conventional thermal recording, a greater force and more heat are exerted on the protective film of the thermal head, making wear and corrosion (or wear due to corrosion) more likely to progress.
With a view to preventing the wear of the protective film on the thermal head so as to improve its durability, a number of techniques have been considered in order to improve the performance of the protective film. Among others, a carbon-based protective film (hereinafter referred to as a carbon protective layer) is known as a protective film excellent in resistance to wear and corrosion.
Thus, Examined Published Japanese Patent Application (KOKOKU) No. 61-53955 discloses a thermal head excellent in wear resistance and response obtained by forming a very thin carbon protective layer having a Vickers hardness of 4500 kg/mm.sup.2 or more as the protective film of the thermal head.
Moreover, Unexamined Published Japanese Patent Application (KOKAI) No. 7-132628 discloses a thermal head which has a dual protective film comprising a lower silicon-based compound layer and an overlying diamond-like carbon layer, whereby the potential wear and breakage of the protective film are significantly reduced to ensure that high-quality image can be recorded over an extended period of time.
These carbon protective layers have a very high hardness and chemical stability, hence they exhibit sufficiently excellent properties to prevent wear and corrosion which may be caused by the sliding contact with thermal materials.
However, the carbon protective layers are not enough to resolve the aforementioned recording unevenness due to the resistance variation of the heating elements with the passage of time. It is also important the carbon protective layers have excellent properties in order to record high quality thermal images having a reduced deterioration with the passage of time over an extended period of time.